Hi Roberto,
Could you confirm if what you meant by getting the client list is to get all the client accounts under your MCC? If yes, then you may do so using the
ManagedCustomerService.get and you may also refer to the
GetAccountHierarchy sample code (other languages are also available in the link).
As for the "AdWordsUser not existing anymore" issue, could you provide more details (any error messages or SOAP request and response generated) so I may also identify the exact issue and the appropriate solution? You may reply using Reply privately to author.
Thanks and regards,
Peter
AdWords API Team